Achromatic flat lens cuts imager size, weight

Article By : EDN Asia

Harvard scientists' metalens is the first to work across a continuous blue-to-green bandwidth from 490nm to 550nm without chromatic dispersion.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed the first flat lens that works within a continual bandwidth of colours, from blue to green. This bandwidth, close to that of a typical LED, paves the way for new applications in imaging, spectroscopy and sensing.

“Traditional lenses for microscopes and cameras — including those in cell phones and laptops — require multiple curved lenses to correct chromatic aberrations, which adds weight, thickness and complexity,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering. “Our new breakthrough flat metalens has built-in chromatic aberrations corrections so that a single lens is required.”

Flat lens technology breakthrough

One of the major challenges in developing a flat, broadband lens has been correcting for chromatic dispersion, the phenomenon where different wavelengths of light are focused at different distances from the lens.
 

Figure 1: SEAS researchers have developed the first flat lens that works within a continual bandwidth of colours, from blue to green. This bandwidth, close to that of an LED, paves the way for new applications in imaging, spectroscopy and sensing. (Courtesy: Vyshakh Sanjeev/ Harvard SEAS).
 

Correcting for chromatic dispersion — known as dispersion engineering — is a crucial topic in optics, and an important design requirement in any optical systems that deals with light of different colours. The ability to control the chromatic dispersion of flat lenses broadens their applications and introduces new applications that have not yet been possible.

To design an achromatic lens — a lens without chromatic dispersion — a team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) optimized the shape, width, distance, and height of the nanopillars that make up the heart of the metalens. As in previous research, the researchers used abundant titanium dioxide to create the nanoscale array.

This structure allows the metalens to focus wavelengths from 490nm to 550nm, basically from blue to green, without any chromatic dispersion.

"By harnessing chromatic aspects, we can have even more control over the light,” said Reza Khorasaninejad, a Research Associate in the Capasso Lab and first author of the paper. “Here, we demonstrate achromatic flat lenses and also invent a new type of flat lens with reverse chromatic dispersion. We showed that one can break away from the constraints of conventional optics, offering new opportunities only bound by the designer’s imagination."
 

Figure 2: A scanning electron microscope image shows a side-view of the metalens, with nanopillars optimised to focus colours without chromatic dispersion. Scale bar: 200 nm. (Image courtesy of the Capasso Lab/Havard SEAS).
 

“This method for dispersion engineering can be used to design various ultrathin components with a desired performance,” said Zhujun Shi, a PhD student in the Capasso Lab and co-first author of the paper. “This platform is based on single step lithography and is compatible with high throughput manufacturing technique such as nano-imprinting.”

The researchers worked further to demonstrate an application: a handheld spectrometer that competes with larger benchtop instruments on performance.

 
Next: Metalens enables portable spectrometer »

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